Heretofore, respiratory phase determination apparatuses are known which determine a respiratory phase based on a breath sound of an animal that performs pulmonary respiration. A breath sound is a sound generated as a result of respiration. A respiratory phase includes an inspiratory phase in which the air is taken into the lung and an expiratory phase in which the air is discharged from the lung, and these two phases are normally alternately repeated. Note that the respiratory phase determination apparatuses are used, for example, for lung sound analysis apparatuses for asthma-related tests.
A common operation of a respiratory phase determination apparatus is as follows. The respiratory phase determination apparatus first extracts a respiratory periods each divided by a pause between inspiration and expiration from a breath sound of a person to be measured (hereinafter referred to as “subject”), and feature values. The respiratory phase determination apparatus determines whether a respiratory period is an inspiratory phase (also referred to as “inspiration period”) or expiratory phase (also referred to as “expiration period”) depending on whether or not the feature value meets a predetermined datum (conditional expression). This determination is referred to as “respiratory phase determination” hereinafter.
The above-described feature values can be roughly divided into the following three categories.
A feature value using the magnitude of power or amplitude as a datum belongs to the first category. As such feature values, for example, power of a breath sound of the chest (NPL 1), power of a breath sound of the neck (PTL 1, NPL 2), and maximum amplitude of a breath sound of the neck (PTL 1) or the like are known.
A feature value using the length of a respiratory period as a datum belongs to the second category. As such feature values, for example, length of a respiratory period (PTL 1, NPL 2), ½ Gaussian width (PTL 1) are known.
A feature value using the rate of change of power in a short time as a datum belongs to the third category. As such feature values, for example, power in the first-half part of a breath sound of the neck (NPL 2), difference in power between the first ⅓ part and the last ⅓ part of a breath sound of the neck (NPL 2), angle of inclination at a rising edge (PTL 1), and angle of inclination at a falling edge (PTL 1, NPL 2) are known.